All publications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
High value materials, including gold and silver coins and bars, have long been targets for counterfeiters. As such, there has existed a need for a non-destructive method that can ascertain the authenticity of the material composition. Such an approach must not only test the surface of the material, but must also assess its unseen interior as well.
One of the oldest methods to determine the composition of materials is the so-called “Archimedes method,” which evaluates the volume and mass of an object to determine its density. Since gold has an abnormally high density (19.30 g/cm3), this approach has been used to evaluate the authenticity of gold coins and bars. However, there are other materials with a similarly high density, including tungsten (19.25 g/cm3), which is worth roughly 1/1000 the price of gold, and also depleted uranium (19.05 g/cm3). These cheap alternatives can be used in the core of gold coins and bars by counterfeiters to devalue the materials, and cannot be detected by the Archimedes method. While these substitutes have slightly lower densities than gold, they can be complimented by the addition of small amounts of heavier, more expensive elements, such as rhenium, platinum, and osmium. In 2010, fake gold bars were discovered in Hong Kong that featured a pure gold exterior with an interior comprised of a complex alloy of gold, osmium, iridium, ruthenium, copper, nickel, iron, and rhodium.
Another approach to nondestructively determine the authenticity of a material is X-ray fluorescence. This method works by analyzing the secondary X-rays from a material that has been bombarded by high-energy X-rays or gamma rays, and can be used very precisely in elemental analysis. Unfortunately, the penetration depth of X-ray fluorescence is small, ranging from μm to mm, thus making it unable to analyze the materials comprising the core of coins and bars.
Ultrasonic testing can also be used for elemental analysis. However, ultrasonic testing requires a large flat surface for the probe to make contact with. In the case of coins, surface features that are irregular in shape make analysis all but impossible. Furthermore, depending on how and where the tungsten is embedded, it may not be detected by ultrasound.
Consequently, there exists a need for nondestructive means to nondestructively determine the authenticity of materials.